Various types of multi-voltage level on-board systems have been proposed. Typically, the voltages can be 12 volts and 24 volts. The 12 volt system is used, in well known manner, to supply the usual load encountered in the vehicle such as headlights, signalling lights, radios and the like. The 24 volt system is particularly adapted to supply power to high power loads, such the starter of the vehicle. The high power users require higher operating currents if the battery voltage drops. High currents cause numerous problems, such as heating of electrical supply lines, high resistance contacts or terminals, and the like. Raising the overall on-board voltage for all loads is not necessarily desirable, however, since the lifetime of many loads decreases with increasing voltage--for example incandescent lamps--and problems with insulation arise, particularly in the presence of moisture. To match the voltage level to the various loads for optimum performance, it has previously been proposed to provide multi-voltage circuits in the on-board network of the vehicle. The normal, usually supplied battery supplies the customary and usual and connected loads of the vehicle. The second battery is added only when loads requiring high power are to be connected. The starter is then supplied from both the main battery and an auxiliary battery, connected in series. This system has difficulties, however, since switch-over of the batteries is required in dependence on the operating system and network of the vehicle. Separate switch-over circuits are needed in order to permit charging of both batteries by the alternator of the vehicle. Normally, the alternators are designed to provide output power only at a single voltage level. Under ordinary operation of the vehicle, a single battery is sufficient. The second series connected battery which is usually used only upon starting of the vehicle is not loaded or needed in normal operation. As far as the charging circuit of the battery system, then, is concerned, the second battery requires charging at a much lower level, and with less energy and at a lower power level than charging of the main battery.
The textbook by Kirdorf, "Praxis der Autoelektrick, Vol IV, Page 195, (1970)," describes a circuit in which two batteries are switched by a battery transfer switch. This system has the disadvantage that a high voltage transfer switch is necessary in order to connect the batteries in accordance with the operating mode of the vehicle. Such transfer or change-over switches are subject to malfunction and are expensive. It has also been proposed to generate a second, higher voltage from the rectified voltage of an alternator of a vehicle by using a voltage doubling circuit. Such a circuit--known as such--can be used since the additional or auxiliary battery requires only a relatively low charging current. Electronic voltage double circuits have the disadvantage, however, that they require modification of the three phase rectifier used with the alternator and that the circuit is comparatively complex and hence expensive.